The silicon carbide (SiC) material has promising applications in high temperature, high frequency, high power, photoelectronics, and anti-radiation and the like fields, due to its advantages such as wide band-gap, high critical break-down electric field strength, high thermal conductivity, and high carrier saturation mobility velocity. Especially, semi-insulating SiC monocrystal substrates have a wide range of applications in microwave devices. For example, SiC-based microwave devices, such as transistors, manufactured on the semi-insulating SiC monocrystal substrates can produce at a frequency of 10 GHz power density five times greater than that of GaAs-based microwave devices. However, the high-performance SiC-based microwave devices require high crystallization quality of the semi-insulating SiC monocrystal substrates. Here, “semi-insulating” refers to a resistivity greater than 105 Ω·cm at the room temperature, and thus is conceptually similar to “high resistivity” in this context.
In theory, intrinsic SiC crystal exhibits the semi-insulating characteristic due to its wide band gap. However, a SiC crystal without intentional doping during growth thereof may have a resistivity of about 0.1-1000 Ω·cm because of impurities such as N and B in the raw SiC material, impurities such as B and Al in the graphite crucible and the thermal insulation, and impurities such as N impurities remaining in ambient environment. Here, we define these unintentional introduced impurities as “intrinsic impurities”. A wafer having such a resistivity range cannot satisfy the requirement for manufacturing the microwave devices.
At present, the semi-insulating SiC crystal with a resistivity greater than 105 Ω·cm is obtained generally by forming a deep energy level in the forbidden band of SiC. The principle underlying this approach is that the resistivity of the material can be improved by introducing the deep energy level into the forbidden band of SiC as a compensation center to capture free carriers. Here, the “deep energy level” refers to an energy level distant from an edge of a valance band or a conduction band by 300 meV or more. However, some elements, such as boron, will also create a shallow energy level. Specifically, the shallow energy level enhances the conductivity of the material, instead of improving the resistivity of the crystal.
More specifically, the approach described above may be implemented in two ways. One way is to introduce point defects to generate a deep energy level to compensate the shallow energy level, so as to achieve the semi-insulating SiC crystal. For example, U.S. Pat. No. 6,218,680 discloses a technique of compensating shallow energy level donor impurities and shallow energy level acceptor impurities with intrinsic point defects. This technique emphasizes that the concentration of heavy metal or transition metal should be as small as possible so as not to impact electric performances of the devices. Especially, the content of vanadium should be smaller than 1014 cm−3 or the detection limit of the secondary ion mass spectrometer. However, it is still unknown how to effectively increase or reduce the concentration of the point defects in the crystal. In practice, the concentration of the point defects in the SiC crystal may be insufficient to compensate the shallow energy level impurities so as to achieve the semi-insulating characteristic required by the microwave devices. Further, some of the point defects may be thermodynamically instable. As a result, the semi-insulating characteristic of the SiC crystal cannot be guaranteed in some special environments. For example, researches show that Si vacancies in the SiC crystal will close up after undergoing high temperature annealing for a relatively long period. As a result, the resistivity of the SiC crystal is reduced, and thus it is impossible to achieve the SiC crystal with the stable semi-insulating characteristic.
The other way is to introduce deep energy level dopants. For example, U.S. Pat. No. 5,611,955 discloses a technique of doping the SiC crystal with a transition element, especially vanadium, as the deep energy level dopants to compensate unintentionally doped impurities such as N and B in the SiC crystal, so as to achieve the semi-insulating SiC crystal. However, introduction of the transition element as the deep energy level dopants into the SiC crystal to achieve the semi-insulating SiC crystal will cause some disadvantages. For example, in case where vanadium is introduced as the deep energy level dopants into the SiC crystal, excessive vanadium will also cause crystal defects. If vanadium has a concentration greater than its solid solubility limit (5×1017 cm−3) in the SiC crystal, educts of vanadium and micropipes will be produced, which deteriorates the crystal quality. On the other hand, the excessive amount of doped vanadium will reduce mobility of electrons in the crystal, which impacts the performances of the microwave devices manufactured therefrom.